An investigation is conducted on optimizing the control allocation for trimmed flight on the compound helicopter. The compound helicopter features a single main rotor,a vectored thrust ducted propeller(VTDP)and lifting wings. Due to the redundant controls for thrust,elevator deflection,and differential and symmetric flap deflection,there is a wide range of trim solutions in forward flight for compound helicopter. A method is developed to calculate optimal trim solutions. Firstly,aerodynamics models for deferent subsystems of the compound helicopter are conducted,which consider the mutual interaction of each part. Secondly,a flight dynamics model is developed based on which the method of trim optimization is performed. Finally,the method is demonstrated using a compound helicopter UH-60L/VTDP. The trim optimization of flight conditions from hover to 370 km/h is conducted using the optimization method. The controls,fuselage attitudes as well as the allocation of lift and thrust along with the flight speed are obtained.